Various processes are known for the production of higher linear alpha olefins (for example D. Vogt, Oligomerisation of ethylene to higher α-olefins in Applied Homogeneous Catalysis with Organometallic Compounds Ed. B. Cornils, W. A. Herrmann Vol. 1, Ch. 2.3.1.3, page 245, VCH 1996). These commercial processes afford either a Poisson or Schulz-Flory oligomer product distribution. In order to obtain a Poisson distribution, no chain termination must take place during oligomerisation. However, in contrast, in a Schulz-Flory process, chain termination does occur and is independent from chain length. The Ni-catalysed ethylene oligomerisation step of the Shell Higher Olefins Process (SHOP) is a typical example of a Schulz-Flory process.
In a Schulz-Flory process, a wide range of oligomers are typically made in which the fraction of each olefin can be determined by calculation on the basis of the so-called K-factor. The K-factor, which is indicative of the relative proportions of the product olefins, is the molar ratio of [Cn+2]/[Cn] calculated from the slope of the graph of log [Cn mol %] versus n, where n is the number of carbon atoms in a particular product olefin.
The K-factor is by definition the same for each n. By ligand variation and adjustment of reaction parameters, the K-factor can be adjusted to higher or lower values. In this way, the process can be operated to produce a product slate with an optimised economic benefit. When demand for the C6–C18 fraction is much higher than for the C>20 fraction, processes are geared to produce the lower carbon number olefins. However, the formation of the higher carbon number olefins is inevitable, and, without further processing, the formation of these products can be detrimental to the profitability of the process. To reduce the negative impact of the higher carbon number olefins and of the low value C4 fraction in such processes, additional technology has been developed to reprocess these streams and convert them into more valuable chemicals such as internal C6–C18 olefins. However, this technology is expensive both from an investment and operational point of view and consequently adds additional cost. Therefore, considerable effort is directed to keep the production of the higher carbon numbered olefins to the absolute minimum, i.e. not more than inherently associated with the Schulz-Flory K-factor.
In WO-A-99/02472 novel Fe-based ethylene oligomerisation catalysts are described that show high activity and high selectivity towards linear alpha olefins. The catalysts are based on iron complexes of a selected 2,6-pyridinedicarboxaldehyde bisimine or a selected 2,6-diacylpyridine bisimine. In the present invention the term “bis-(aryliminoalkyl)pyridine”, or in short, “bis-aryliminepyridine” is used to describe both classes of ligands. In WO-A-99/02472, the oligomer product distribution made with these catalysts is not specified any further, but is implied to be Schulz-Flory in view of the definition, the use, and the determination of the Schulz-Flory K-factor. In other publications, such as A. M. A. Bennett Chemtech 1999 July, page 24–28; and references mentioned therein, the product composition was stated to obey a Schulz-Flory distribution. The accompanying experimental data in WO-A-99/02472, however, shows that these catalysts afford a product slate with a surprisingly large amount of heavy products. It has been confirmed that the disclosed oligomerisation catalysts afford a product composition which, in comparison with a Schulz-Flory distribution, contains indeed significantly more heavy products than expected.
Indeed, Table 1 on page 30 of WO-A-99/02472 gives an overview of ethylene oligomerisation experiments catalysed by four different iron complexes (X–XIII). Experiment numbers 16 and 17 of this Table, in which iron complex XI is being used at ethylene pressure of 1.4 MPa (gauge) or 1.5 MPa (15 bar(a)) and 2.8 MPa (gauge) or 2.9 MPa (29 bar(a)) respectively, both give rise to a Schulz-Flory K-factor of 0.79, as derived from the C16/C14 ratio. If it is assumed that a perfect Schulz-Flory distribution is obtained in these experiments, i.e. Cn+2/Cn=K=0.79, it can be calculated that the C30–C100 fraction is 15% wt and the C20–C28 fraction is 21% wt on total product. If it is further assumed that the solids mentioned in Table 1 contain the C20–C100 fraction then this should amount to 36% wt on total product. This should be considered as a maximum solids content since at least the major part of the lowest ethylene oligomers in this fraction remain dissolved in the toluene-solution of the C4–C18 fraction. In Experiment numbers 16 and 17 of Table 1, however, the amount of solids isolated are 14.1 g and 18.0 g, which may be calculated as a solids content of 45% wt and 58% wt on total product, respectively.
Similarly, for a K-factor of 0.81 it can be calculated that the C20–C28 fraction and the C30–C100 fraction are 22% wt and 20% wt on total product, respectively, or maximally 42% wt for the solids content. For Experiment number 18 in Table 1, also using iron complex XI, but now at pressure of 0 MPa (gauge), i.e. 0.1 MPa (1 bar(a)), the amounts of solids isolated are 2.7 g , which may be calculated as a solids content of 54% wt on total product.
The distributions obtained in Experiment numbers 16–18 in Table 1 of WO-A-99/02472 clearly indicate that larger quantities of higher carbon number products, i.e. solids (>C20), are produced than would be expected on the basis of the Schulz-Flory K-factor.